1. Field of the Invention
This invention relates to visual display terminal (VDT) systems, and more particularly relates to VDT systems which employ cathode ray tubes (CRTs). Information may be generated on CRTs by raster scanning, by XY refresh or XY storage techniques. The present invention is concerned with generating alphanumeric characters employing television raster scanning and employing commercially available character generators which generate a dot-matrix defining characters in American Standard Code for Information Interchange (ASCII).
Heretofore, VDTs were well known and had been produced employing stripped down commercially available television chassis. Commercial entertainment television chassis which were employed for VDTs suffer several deficiencies. If less than all the scan lines were employed to complete a frame or picture, the characters displayed on the CRT were not well defined and/or suffered from noticeable decay of brightness which is called flicker. Heretofore, when all of the scan lines were employed to produce a dot-matrix character and the dots are tightly spaced, the character generator and the associated circuitry became large and expensive. When a standard commercial television raster scan circuit is modified to increase the frequency or the number of scan lines, the cost of the CRT controls is greatly increased. Whenever a more dense dot matrix character generator is employed with a greater number of raster scan lines, the dot-matrix character generator and associated circuitry is made more expensive.
Accordingly, there is an unfulfilled need for a VDT which is extremely simple, relatively cheap and provides high resolution and high intensity flicker free display of alphanumeric characters for use with data processing systems.